Much of the route is now paralleled by the M4 motorway, which carries the bulk of long distance traffic in this corridor, leaving the A4 primarily for local traffic, though sections in London and Bristol particularly are still major through routes.

The A4 has gone through many transformations through the ages from pre-Roman routes, Roman roads (such as the one passing Silbury Hill),[2] and basic wagon tracks. During the Middle Ages, most byways and tracks served to connect villages with their nearest market town. A survey of Savernake Forest near Hungerford in 1228 mentions "The King’s Street" running between the town and Marlborough. This street corresponded roughly with the route of the modern A4. In 1632, Thomas Witherings was appointed Postmaster of Foreign Mails by Charles I. Three years later, the king charged him with building six "Great Roads" to aid in the delivery of the post, of which the Great West Road was one.[3]

It was not until the 17th century that a distinct route between London and Bristol started to resemble today's road. During the 17th century, the A4 was known as the Great Road to Bristol. When Queen Anne started patronising the spa city of Bath, the road became more commonly known as Bath Road. Over the years, the direction of the road has taken many detours depending on such factors as changes in tolls or turnpike patronage. For example, in 1750 the toll road from London was altered to go through Melksham;[4] and in 1695 the map maker, John Ogilby, produced a map of the Hungerford area of the Great West Road showing two possible routes.[3]

As Bath became more popular with the wealthy and famous, it was inevitable that turnpike trusts would be set up under the terms of the Turnpike Acts to pay for maintenance and improvements to the road. The first turnpike on this road was between Reading and Theale in 1714.[5] Due to increasing traffic, sections of the road between Kensington, over Hounslow Hill, to Twyford were turnpike by 1717 with the remaining sections placed under turnpike trusts.

Strip Map of Bath Road 1786.

As turnpike trusts were individually run, there was the possibility for greatly differing road conditions, especially over the London Clay basin of Kensington, Brentford, Hounslow and Slough, where winter conditions left the way muddy and uneven. This was not always the case with the Bath Road, as many of the wealthy landowners along the route co-operated informally and exercised a large amount of control over feeder roads. As a result, control of the Bath Road was easy to maintain and many inns and towns became prosperous.

Tollhouses were established at Colnbrook, Maidenhead, Twyford, Castle Street Reading, Thatcham and Benham.[6] During the 1820s, the employment of good surveyors improved the condition of the road and aided an increased flow of wealthy travellers. The tolls raised from such clientele ensured that when the turnpike trusts handed over the route to local highway boards, they had no financial liabilities.[7] Justices of the Peace were empowered by the 1862 Rural Highways Act to combine turnpike trusts into Highways Districts. This meant that by the late 1860s trusts were either not renewing their powers or were being terminated by General Acts of Parliament. For example, most turnpikes in Berkshire, including the Bath Road, were officially wound up by 1878 when legislation transferred responsibility for dis-enturnpiked roads to the new county councils. The tollgate on the Bath Road west of Reading was removed in 1864 as the outward pressure of urban development made rates a more acceptable way of financing the maintenance of what was now a suburban road.

With the improvement being made to the road systems, the business of moving mail became easier and thus more profitable as volumes were able to increase. In Bristol, a postal office had been well established by the 1670s.[8] The journey time to London at this period was about 16 and three quarter hours.[9] A letter from Bath in 1684 took about 3 days going via a postal office in Marshfield on the Bristol Road. (The route to Bristol did not yet go through Bath at this time). Journey times during the Turnpike era fell with the improvements from 2 days in 1752 to 38 hours in 1782 and 18 hours by 1836. Royal Mail coaches in 1836 were able to do the trip in 12 to 13 hours.[3]

Further improvements to regional post services were made between 1719 and 1763 due to contracts with the London Inland Letter Office negotiated by Ralph Allen, the postmaster of Bath.[10]

In the early part of the 19th century, coaching was at its height with six stagecoaches each day carrying passengers to and from London along the Bath Road in 1830,[11] rising to ten by 1836.[3] Hungerford is at about the midway point of the journey between London and Bristol and was ideally positioned to take advantage of the increase in coaching. In 1836, five companies operated a coaching service through Hungerford.[3] This peak was to be short-lived following the construction of the Great Western Railway. The decline in coaching traffic in Hungerford coincided with the building of the Great Western Railway from London to Bath and Bristol, and the subsequent Berks and Hants Railway line from Newbury to Hungerford itself in 1847.[3] By 1843, it was reported that the stage coaches had ceased running between Bristol and London.[12]

The road then passes the Glaxo-Smith Kline premises in Brentford where it follows underneath its successor the M4 as far as the bridge over the River Brent. The stretch between Chiswick's western border to Syon Lane (Gillette Corner) is known as the Golden Mile with some notable Art Deco factories. The road remains as the Great West Road through Osterley and Hounslow where it splits with the A30, which is known as the Great South West Road. Between the two world wars, the Great West Road was built as a bypass to relieve traffic congestion in Brentford and Hounslow.

After leaving Heathrow Airport, still the A4, it becomes the Colnbrook Bypass and climbs over the M25 motorway. This bypass for the narrow main road of Colnbrook was built after the Second World War across farmland between Harmondsworth and the outskirts of Langley.

The next major intersection is junction 5 of the M4 (Slough-East) known as the Langley junction. At this point the A4 loses its trunk road classification.

Continuing towards Slough town centre, the road, now named London Road, passes Kedermister Park on the right. Changing into Sussex Place, the architecturally impressive St Bernard's former convent is on the right. Becoming the dual carriageway Wellington Street (a late 1960s bypass for the High Street), Tesco's massive 'aircraft hangar' supermarket is on the right and the Queensmere and Observatory Shopping Centres on the left.

The end of this section of the Great Western Road is in the centre of Slough at the junction of William Street and Wellington Street. This spot was formerly the site of Slough's largest roundabout on the south-east corner of Thames Valley University. Nearby is Slough railway station, served by Great Western Railway.

The roundabout was removed in 2011 and the university's Slough campus closed in 2011 after planning permission was granted for 1,598 flats.

Continuing from the William Street / Wellington Street junction, in the centre of Slough, the road heads westwards towards Maidenhead. The short stretch of dual carriageway is Wellington Street, then briefly on the most western end of the High Street, over the railway bridge for the line between Slough and Windsor Central Station.

The Bath Road begins on the west side of the bridge. On the left is Slough's 1936 Town Hall, now abandoned by the local council, and on the right Salt Hill Park.

The next major road intersection is by the Three Tuns Pub where the A4 crosses the A355. This road goes north towards Farnham Royal, Farnham Common, Beaconsfield and the M40 at junction 2. To the south, it goes towards the M4 (junction 6, Slough Centre) and Windsor.

On Slough's western boundary is the Huntercombe Spur roundabout linking the A4 to junction 7 of the M4 motorway. Here the dual carriageway ends. The road bends right at Taplow and passes under a high railway bridge, then over the River Thames flood prevention channel.

Along Bridge Road, St Cloud Way and Bad Godesberg Way respectively. The centre of Maidenhead is reached at the roundabout that forms the junction with the A308. Turning northwards on the A308 would lead to the A404 towards Marlow and High Wycombe. Southwards, the A308 heads towards Windsor and provides an opportunity to branch off for junction 8/9 of the M4.

Heading out of Maidenhead town centre, the road follows Castle Hill and then Bath Road. At the outskirts, the road forms part of junction 9b of the A404(M)/A404, which links junction 8/9 of the M4 with junction 4 of the M40 at Handy Cross. The A4 crosses open countryside before following New Bath Road on the outskirts of Twyford. This is a bypass, built in 1929, that deviates from the old route of the A4, and crosses the River Loddon on its way into the suburbs of Reading via the village of Charvil.

The A4 passes the King George's Field in Sonning, which are used as playing fields, and the Reading Cricket and Hockey Club. As it enters Reading, along the London Road again, it crosses the junction of the A3290 next to Palmer Park, which leads to Junction 10 of the M4.

The A4 goes over the intersection with the A329Cemetery Junction which links Wokingham with Pangbourne, passing the Royal Berkshire Hospital and the London Road Campus of the University of Reading. At London Street and again at Southampton Street, the A4 meets the A327 twice, since the latter loops round on two branches which meet at Whitley Street. Between the two junctions, the A4 becomes Crown Street for approximately 150 metres.

On the other side of the intersection the A4 follows Pell Street and continues westward. Here it crosses the River Kennet, the Holy Brook and the A33 relief road, which goes towards Basingstoke. In West Reading, the A4 becomes Berkeley Avenue, and then continues onto the Bath Road. It passes Prospect Park, and the suburbs of Southcote, Horncastle and Calcot, before reaching Junction 12 of the M4. The A4 heads south westerly through Theale and over the roundabout connecting it to the A340 which heads towards Pangbourne.

Heading towards Thatcham, the road passes Aldermaston Wharf next to the Kennet and Avon Canal and through the villages of Woolhampton and Midgham. A large industrial estate is on the left as the road enters Thatcham. At the roundabout after the industrial Eestate the route follows London Road and passes Thatcham towards Benham Hill and Newbury. At the Benham Hill roundabout it goes along Bath Road where it deviates from Turnpike Road. Bath Road is the original turnpiked road and the road now confusingly called Turnpike Road was originally called The Shaw Road.[13] The turnpike then continues along London Road as it enters the outskirts of Newbury.

Heading towards the centre of Newbury, the route passes the West Berkshire Community Hospital on the right. To the North of the town centre the road becomes part of an unusual junction with the A339 that links the new A34 by-pass to the north with Basingstoke to the south. The junction is a combination of a figure of eight roundabout with an elevated section that used to be the old route of the A34.

After the junction, the road heads out of town though Speenhamland along Western Avenue and Bath Road to the junction with the A34 Newbury bypass. The Newbury bypass was opened, after some controversy, on 17 November 1998, at a cost of £104 million.[14] The route heads over undulating countryside in a fairly straight westward direction towards Hungerford parallel to the Kennet and Avon Canal and crossing the River Kennet on the outskirts of the town at the same time as intersecting with the A338. The route does not enter the town centre, but continues westward to the northern side of town on its way to Marlborough via the village of Froxfield and the northern edge of Savernake Forest.

On entering the outskirts of Marlborough the road follows London Road. Crossing the River Kennet again it briefly shares the highway with the A346 which connects northwards to Swindon. It splits again at a roundabout along New Road and Oxford Street into the town centre on the High Street. The High Street is a traditional main thoroughfare for an English market town, in that it is wide, with space for market stalls on either side and in the middle, though these spaces are usually marked for car parking.[15]

Until the late 1980s most of this section, and some of the following Wiltshire section, was configured as three lanes, with the central lane available for overtaking traffic in either direction. This is no longer considered safe so the road now has one wider-than normal lane in each direction, with ghost islands at junctions to discourage overtaking at these points.

The A4 continues along the High Street on its way out of Marlborough via Bridewell Street and Bath Road. The road passes underneath a covered bridge that links Morris House of Marlborough College to the North Block of the same establishment.[16] The college is an independent boarding school established in 1843 using some of the buildings that remained after the demise the coaching trade, which saw the original Castle Inn Coaching House close.

The route continues westward through the village of Fyfield, across Overton Hill where there is a parking area at the start of the Ridgeway National Trail and through the village of West Kennet. On leaving West Kennet there are some parking lay-bys where visitors can walk a short distance from the road to the Neolithic West Kennet Long Barrow, which forms part of the Avebury World Heritage Site.[17] One mile further along the A4 is Silbury Hill, which is also part of the Avebury World Heritage Site. A purpose built car park is located beyond the hill on the right travelling westward.[18]
As the route approaches the Beckhampton roundabout, which forms the intersection with the A361, it passes by the Waggon & Horses Inn, built in 1669 to profit from the increasing trade along the old Bath Road. It is mentioned in The Pickwick Papers novel by Charles Dickens.[19]

The route continues past Cherhill and the Cherhill White Horse[20] can be seen to the left and nearby the Lansdowne Monument. The route then descends down the Labour In Vain Hill through the village of Quemerford and into the market town of Calne.
Heading towards the town centre, the road crosses a double mini roundabout, one exit forming the junction with the A3102 to Melksham. It follows New Road and passes by the old coaching inn of the Lansdowne Strand.[21] On the way out of Calne, the road goes along Curzon Street, and then to Chilvester Hill at the roundabout where the northern part of the A3102 splits off towards Lyneham.

The A4 descends steeply before climbing again up Black Dog Hill past the Bowood House Estate. The old road used to pass through Derry Hill, but the New Road, as it is called at this point, indicates that the village was bypassed. There follows a steep descent to the junction with the A342Devizes road and a sharp right turn past the Lysley Arms towards Chippenham.

On the outskirts of Chippenham is a large roundabout, where the old A4 used to carry straight on down London Road and The Causeway into the Town Centre, which has since been pedestrianised. Most traffic turns left to go round the Pewsham Estate relief road called Pewsham Way, which is now the classified A4 route. After four roundabouts, the A4 turns left in a southerly direction at a roundabout, past the Chippenham Magistrates Court. The route heads downhill over the River Avon on the Avenue La Fleche, named after one of Chippenham’s twin towns, to the Bridge Centre roundabout.

The Bridge Centre roundabout forms a junction of the A4 with the A420 Bristol Road. The route heads over Rowden Hill past the Chippenham Community Hospital and down to the spot where musician Eddie Cochran was killed in a car crash on 17 April 1960. A plaque has been placed there in remembrance.[22]

From there it passes under the railway arch built by Isambard Kingdom Brunel in the 1830s and on to its junction with the A350. From this point onwards it regains its trunk road status and heads up Chequers Hill towards Corsham. The road passes through Pickwick, which used to be a village in its own right. Charles Dickens stayed at the Hare & Hounds Inn and used the village name for the title of his novel The Pickwick Papers.[23]

After a series of roundabouts the A4 continues uphill, past what used to be the Ministry of Defence Naval Operations site at Copenacre (now a housing development) before descending steeply to the village of Box. From the road the Box Tunnel, built as part of the Great Western Railway, can be seen clearly. The road bears to the right at the junction with the A365 to Melksham, and criss-crosses with the main railway line as far as the roundabout at Bathford which forms a junction with the A363 towards Bradford-on-Avon. The road after this roundabout becomes a dual carriageway until the A4 diverts to the left down a slip road, and the remainder becomes the A46; this section, the 3-mile (4.8 km) £45 million dual-carriageway Batheaston/Swainswick Bypass, opened in summer 1996, was highly controversial both at planning and at construction stages.

The route enters the outskirts of Bath passing by the training grounds of Bath RFC. After a series of traffic light controlled junctions, the A4 continues straight ahead where it meets the A36. It passes Hedgemead Park and down The Paragon before crossing into George Street and to Queen Square.

In the original 1922 road numbering list, the A4 ended at the junction between George Street and Milsom Street in Bath. On 1 April 1935, it was extended over the A431 and B4044 to Newbridge, and the A36 to Avonmouth.[24][25]

From Queen Square, Bath, the A4 carries on to Queen Square Place and Charlotte Street, before heading along the Upper Bristol Road by Royal Victoria Park to Newbridge Road. The road crosses the River Avon on its way to join the A36 and become a dual carriageway again until the roundabout that forms the junction with the A39 Wells Road at the Globe Inn and exit for the nearby Bath Spa University. It continues on through the village of Saltford towards Keynsham where a roundabout splits the road and the A4 continues as a dual carriageway by-passing the town, crossing over the River Chew in the process.

The end of the by-pass coincides with the junction of the Avon Ring Road, the A4174. The route goes though the suburb of Brislington, going past Arnos Court Park and Arnos Vale Cemetery. The road runs over a bridge over the New Cut and into Temple Gate where Bristol Temple Meads railway station is located. The A4 no longer has a defined route from this point to the start of Anchor Road at The Centre; originally it went along Victoria Street and Baldwin Street before turning left onto St Augustine's Parade or, later, Broad Quay, but in more recent times it followed Redcliffe Way, via Redcliffe Roundabout (junction with the A38) across Queen Square to The Centre. From there it turns back towards Bristol Harbour via Anchor Road past the We The Curious science museum. After another roundabout the A4 follows along Hotwell Road and around a one-way system that merges the A4 with the A3029.